A conventional write driver circuit will be described with reference to FIG. 9. In the write driver circuit, as illustrated in FIG. 9, the collectors of NPN transistors Q21 and Q22 are connected to a power input terminal (Vcc), the collector of an NPN transistor Q23 is connected to the emitter of the NPN transistor Q21, the collector of an NPN transistor Q24 is connected to the emitter of the NPN transistor Q22, the emitters of the NPN transistors Q23 and Q24 are connected to each other, and a current source I1 is connected between the emitters of the NPN transistors Q23 and Q24 and the ground terminal.
A resistor R25 is connected between the collector and base of the NPN transistor Q21, a resistor R26 is connected between the collector and base of the NPN transistor Q22, the collector of an NPN transistor Q25 is connected to the base of the NPN transistor Q21, the collector of an NPN transistor Q26 is connected to the base of the NPN transistor Q22, the emitters of the NPN transistors Q25 and Q26 are connected to each other, and a current source I2 is connected between the emitters of the NPN transistors Q25 and Q26 and the ground terminal.
The bases of the NPN transistors Q23 and Q25 are connected to each other, the bases of the NPN transistors Q23 and Q25 are provided with an input terminal WD of a write signal, the bases of the NPN transistors Q24 and Q26 are connected to each other, and the bases of the NPN transistors Q24 and Q26 are provided with an input terminal WDB of a write signal.
In the write driver circuit, one (X) of terminals of a magnetic head HD is connected to the connection point of the NPN transistors Q21 and Q23 and the other terminal Y of the magnetic head HD is connected to the connection point of the NPN transistors Q22 and Q24.
In the write driver circuit having such a construction, write signals of opposite phases are supplied to the input terminals WD and WDB. For example, when a high-level (H) write signal is supplied to the input terminal WD and a low-level (L) write signal is supplied to the input terminal WDB, the NPN transistors Q21 and Q24 are turned off, the NPN transistors Q22 and Q23 are turned on, and a current is passed to the magnetic head HD in the direction from the terminal Y to the terminal X. When the states of the write signals are opposite, the NPN transistors Q21 and Q24 are turned on, the NPN transistors Q22 and Q23 are turned off, and a current is passed to the magnetic head HD in the direction from the terminal X to the terminal Y.
The conventional write driver circuit has, however, drawbacks as described hereinbelow caused by the fact that the magnetic head HD has an inductance. Specifically, a counter electromotive force occurring across the magnetic head HD when the current passing through the magnetic head HD is reversed is expressed by the following expression (1) where the current passing through the magnetic head HD is i, the counter electromotive force (voltage across the head terminals) occurring across the magnetic head HD is V, the inductance of the magnetic head HD is L, and t denotes time. EQU V=L(di/dt) (1)
From the relation of Expression (1), time required for the current passing through the magnetic head HD to be reversed is proportional to each of the inductance L of the magnetic head HD and the current passing through the magnetic head HD and is inversely proportional to the counter electromotive force V. Consequently, the larger the counter electromotive force V is, the shorter the time required for the current passing through the magnetic head HD to be reversed is, because the energy accumulated by the inductance L of the magnetic head HD is discharged in accordance with the product of the level of the counter electromotive force and time.
The operation in a transition period in which, for example, an H-level write signal is supplied to the input terminal WD in a state where an L-level write signal is supplied to the input terminal WD and an L-level write signal is supplied to the input terminal WDB, the state of the write signal is changed to a state in which the H-level write signal is supplied to the input terminal WDB, and the direction of the write current passing through the magnetic head HD is changed from the direction from the terminal X to the terminal Y to the direction from the terminal Y to the terminal X will now be described.
In the write driver circuit, when the state of the write signal is reversed, the H-level write signal is supplied to the input terminal WD, and the L-level write signal is supplied to the input terminal WDB, the NPN transistor Q25 is turned on and a current of the current source I2 is passed to the resistor R25, so that the base potential of the NPN transistor Q21 is decreased from the power source voltage by an amount corresponding to a voltage drop caused by the resistor R25 and the current source I2. Since the NPN transistor Q21 is not completely turned off at this time, that is, remains in the on-state, a voltage Vx at the terminal X of the magnetic head HD is dropped from the base potential of the NPN transistor Q21 only by a base-emitter voltage Vbe of the NPN transistor Q21.
On the other hand, when the NPN transistor Q26 is turned off, the base potential of the NPN transistor Q22 is pulled up to the power source voltage and the NPN transistor Q22 is turned on. A voltage Vy at the terminal Y of the magnetic head HD is consequently dropped from the power source voltage only by the base-emitter voltage Vbe of the NPN transistor Q22.
From the above, when the power voltage is Vcc, the voltage difference between the terminals X and Y of the magnetic head HD is expressed by the following expression (2). EQU .vertline.Vy-Vx.vertline.=.vertline.(Vcc-Vbe)-(Vcc-I2.multidot.R25-Vbe).ver tline.=I2.multidot.R25 (2)
This similarly applies to the case where the reversing direction of the current passing through the magnetic head HD is opposite and the voltage difference between the terminals X and Y of the magnetic head HD in this case is expressed by the following expression (3). EQU .vertline.Vy-Vx.vertline.=.vertline.(Vcc-I2.multidot.R26-Vbe)-(Vcc-Vbe).ver tline.=I2.multidot.R26 (3)
As described above, since the voltage difference occurring between the terminals X and Y of the magnetic head HD in a transient period is determined by the resistor R25 or R26 and the current source I2, the counter electromotive force generated by the magnetic head HD is regulated by the voltage difference occurring between the terminals X and Y of the magnetic head HD.
In order to reverse the current passing through the magnetic head HD at high speed, it is necessary to widen the voltage difference between the terminals X and Y of the magnetic head HD so that the counter electromotive force generated by the magnetic head HD is not regulated. For this purpose, it is preferable to widen the voltage difference between both terminals of the magnetic head HD by increasing the current of the current source I2 while not increasing the value of resistance of each of the resistors R25 and R26 for the following reason. When the value of resistance is increased, the area of the resistance region becomes large, so that high packing density cannot be achieved. Moreover, the parasite capacity by the resistance region comes to be unignorable and it causes a slow current reversal.
A state after the reversal of the write current passing through the magnetic head HD from the direction from the terminal X to the terminal Y to the direction from the terminal Y to the terminal X is finished will now be considered. In this case as well, the high-level write signal is continuously supplied to the input terminal WD and the low-level write signal is continuously supplied to the input terminal WDB, the NPN transistors Q21 and Q24 are turned off, the NPN transistors Q22 and Q23 are turned on, and the direction of the write current passing through the magnetic head HD is from the terminal Y to the terminal X.
At this time, the voltage Vy at the terminal Y of the magnetic head HD is dropped from the base potential of the NPN transistor Q22 only by the base-emitter voltage Vbe of the NPN transistor Q22. The voltageVx at the terminal X of the magnetic head HD is dropped from the voltage Vy at the terminal Y only by an amount of a voltage drop caused by the write current passing through the magnetic head HD and the resistance component of the magnetic head HD.
The voltage drop caused by the write current I1 passing through the magnetic head HD and a resistance component RH of the magnetic head HD is expressed by the following expression (4) EQU .vertline.Vy-Vx.vertline.=I1.multidot.RH (4)
This similarly applies to a case where the reversing direction of the write current passing through the magnetic head HD is opposite. That is, when the reversal of the write current I1 passing through the magnetic head HD is finished, the voltage difference between both terminals of the magnetic head HD is determined only by the write current I1 passing through the magnetic head HD and the resistance component RH of the magnetic head and the voltage drop by the resistors R25 and R26 does not exert any influence.
From the above, it is necessary to pass a relatively large current to the current source I2 in order to reverse the write current passing through the magnetic head HD at a high speed. When the reversal of the write current passing through the magnetic head HD is finished, however, the relatively large current flowing to the current source I2 becomes useless.